Fluid hydrocarbons, such as petroleum, natural gas, and other hydrocarbons, may be obtained from a subterranean geologic formation or strata by drilling a borehole or wellbore that penetrates the formation. While the wellbore provides a partial flowpath for the hydrocarbons to the surface, it will be recognized that, for the hydrocarbons in the formation to be produced in paying quantities, the hydrocarbons initially must flow substantially unimpeded in the formation to the wellbore. Absent damage to the formation during drilling or well completion operations, the rate of flow of the hydrocarbons to the wellbore will be determined in large measure by the permeability of the matter, i.e., rock, comprising the formation.
For a variety of reasons, formation permeability, at least in some segments or areas of the formation, may be insufficient to provide the flow desired. For example, as indicated, drilling fluids and other fluids employed during drilling or development operations may penetrate the formation and cause plugging of the pores of the rock comprising the formation. In addition, during production, fine particles migrating with hydrocarbons through the formation may cause plugging of some formation pores. Finally, some formations are comprised of rock having few and/or very small pores, so that the rate of hydrocarbon flow, even under the best of circumstances, is slow.
To provide or insure an acceptable rate of flow of the hydrocarbons to the wellbore, various formation treatment procedures, referred to collectively as well stimulation, have been developed. For example, “fracturing” and “acidizing” are common oilfield stimulation techniques, the first-mentioned technique involving operations directed to providing an actual cleft or fracture in the rock through which the hydrocarbons may flow, while “acidizing” involves injection of acid(s), e.g., mineral acids, into the formation for reacting with components in the rock and/or for cleaning the pores. However, as is well recognized, fracturing can damage a formation, and use of acids presents its own set of problems, such as, in the case of strong acids, limited radial penetration of the rock in some instances because of rapid reaction and spending of the strong acid(s). Accordingly, procedures have been developed which provide for injection of different types of reactants (e.g., weaker acids, such as formic acid or acetic acid) into the formation to react with the rock and/or plugging materials and improve permeability at locations distant from the wellbore.
In a doctoral dissertation titled “THE INFLUENCE OF TRANSPORT AND REACTION ON WORMHOLE FORMATION IN CARBONATE POROUS MEDIA: A STUDY OF ALTERNATIVE STIMULATION FLUIDS” (University of Michigan, 1998), C. N. Fredd proposes the use of relatively low concentration solutions prepared by dissolution of specified reagents in water as possible reactants for stimulating calcium carbonate-containing formations. In particular, an aqueous solution prepared with disodium salt dihydrate of ethylenediamine-tetraacetic acid, and similarly prepared solutions of 1,2 cyclohexanediamine tetraacetic acid (CDTA) and diethylene triaminepentaacetic acid (DTPA) were tested in laboratory experiments to determine possible “wormhole” formation in calcium carbonate cores. However, as acknowledged in the dissertation, the effects or results of higher concentrations of the reactants in the solutions, as would be used in field operations, were unknown. All the experiments were done at solution concentrations of about 0.25 molar; for perspective that would be about 8.5 weight percent of disodium EDTA. Moreover, all of the experiments were conducted at relatively low temperatures, which are significantly below many deep well temperatures.
Because all conventional procedures for well stimulation have their own limitations, there exists a continuing need in the oilfield industry for improved methods and techniques for treating subterranean formations to improve permeability. For example, as mentioned, the Fredd dissertation acknowledges that the results of utilizing higher concentrations of the reactants employed in the experiments reported, as would be desired in actual field operations, were unknown. Thus, the suggestion by Fredd of the use of relatively low concentration solutions prepared from disodium dihydrate ethylenediamine-tetraacetate appears somewhat limited, in that the concentrations employed in the Fredd experiments appear to be close to the maximum solubility of the composition at lower pH levels. Accordingly, the invention described hereinafter addresses this continuing need for effective and improved formation stimulation, and provides an improved method for stimulating subterranean geologic formations.